In order to reduce the size of a semiconductor device and achieve high functionality, a semiconductor device having a digital circuit and an analog circuit on one semiconductor substrate has been developed.
In such a semiconductor device, there is a problem that a noise that occurs in the digital or analog circuit propagates through the substrate and influences other circuits. Especially, the noise emitted by the digital circuit propagating to the analog circuit through the substrate, and causing the analog circuit to malfunction can be a big problem.
Technologies to solve this problem are disclosed in Patent Documents 1 and 2.
In the technology disclosed in Patent Document 1, a deep well of the opposite conductivity type to a semiconductor substrate isolates the whole analog circuit part or the whole digital circuit part from the semiconductor substrate.
Hereinafter, explanations will be made assuming the conductivity type of a semiconductor substrate is P-type, however, it may be N-type. In that case, the conductivity type of all the constituent elements will be reversed.
For instance, referring to FIG. 1 of Patent-Document 1, an analog circuit part formed on a P-type semiconductor substrate 10 is isolated from the semiconductor substrate by an N-type deep well 12. Further, a digital circuit part is surrounded by an N-type deep well 14.
In the technology disclosed in Patent-Document 2, in an analog circuit part, a well of the same conductivity type as a semiconductor substrate is surrounded by a deep well of the opposite conductivity type to the semiconductor substrate.
For instance, referring to FIG. 1 of Patent-Document 2, a P-type well 4 in an analog circuit part formed on a P-type semiconductor substrate 1 is isolated from the semiconductor substrate 1 by an N-type deep well 4.
In these technologies, depletion layers formed between the semiconductor substrates and deep wells prevent the propagation of noise.
[Patent Document 1] Japanese Patent Kokai Publication No. JP-A06-163823
[Patent Document 2] Japanese Patent Kokai Publication No. JP-A06-69436